Factors influencing tissue turnover during winter in perennial ryegrass-dominated swards

Despite low net grass growth rates in Ireland between mid-November and February, tissue continuously turns over as new leaves are produced and older leaves senesce. The effects of closing swards from grazing on three dates in autumn (1 September, 20 September and 10 October) on tissue turnover in a perennial ryegrass sward during the winter were examined at two sites in the north-east and south of Ireland. Leaf extension and senescence rates were greater in the south than in the north-east, and were greatest on the earliest closing date at both sites. Leaf appearance and extension rates at both sites were closely correlated with accumulated daily air temperature above 6 degrees C and mean soil temperature. Site-specific factors also influenced tissue flux, possibly including grazing intensity at closing and density of tillers of perennial ryegrass. Swards in the autumn with high herbage masses [> 2000 kg dry matter (DM) ha(-1) approximately] and high leaf area index (LAI; about 2.5 and above) had the greatest leaf senescence rates and the greatest declines in herbage mass during the winter. Leaf senescence rates per tiller in high LAI swards were correlated positively with amount of leaf per tiller and also the proportion of green leaf in the second youngest expanding leaf lost to senescence. As potentially a complex of interactive factors influences tissue flux in winter, it is proposed that the data from this study be used in conjunction with other data to construct a dynamic model to predict more reliably optimum closing date in autumn for herbage utilization in winter.